Water electrolysis system and method for operating the same

ABSTRACT

A water electrolysis system includes a high-pressure hydrogen production unit for electrolyzing water to generate oxygen and high-pressure hydrogen (the pressure of the high-pressure hydrogen being higher than that of the oxygen), and a gas-liquid separation unit for removing water contained in the high-pressure hydrogen. The gas-liquid separation unit is placed on a hydrogen pipe for discharging the high-pressure hydrogen from the high-pressure hydrogen production unit. In addition, the water electrolysis system includes a high-pressure hydrogen supply pipe for transferring dewatered high-pressure hydrogen from the gas-liquid separation unit, a cooling unit, which is placed on the high-pressure hydrogen supply pipe and is capable of variably controlling the temperature of the high-pressure hydrogen to adjust the humidity of the high-pressure hydrogen, and a control unit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Applications No. 2011-188814 filed on Aug. 31, 2011, andNo. 2012-027051 filed on Feb. 10, 2012, of which the contents areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a water electrolysis system, which hasa high-pressure hydrogen production unit for electrolyzing water togenerate oxygen at an anode side and hydrogen at a cathode side, and ahydrogen supply pipe for supplying hydrogen generated in the productionunit. The present invention also relates to a method for operating sucha system.

2. Description of the Related Art

In fuel cells, hydrogen generally is used as a fuel gas for performing apower generation reaction. For example, a water electrolysis apparatusis used to produce hydrogen. The water electrolysis apparatus contains asolid polymer electrolyte membrane (an ion-exchange membrane) fordecomposing water to generate hydrogen (and oxygen). Electrode catalystlayers are formed on either side of the solid polymer electrolytemembrane to thereby prepare a membrane-electrode assembly, and currentcollectors are placed on either side of the membrane-electrode assemblyto produce a unit cell.

A plurality of such unit cells are stacked, a voltage is applied torespective ends of the cell stack in the stacking direction, and wateris supplied to the anode-side current collector. Then, the water isdecomposed to generate hydrogen ions (protons) at the anode side of themembrane-electrode assembly. The hydrogen ions permeate through thesolid polymer electrolyte membrane to the cathode side, and becomebonded with electrons to produce hydrogen. Meanwhile, at the anode side,oxygen generated simultaneously with the hydrogen is discharged togetherwith residual water from the cell stack.

Hydrogen generated by the water electrolysis apparatus contains water. Ahydrogen product for a fuel cell vehicle or the like is required to bein a desired dry state (to have a desired water concentration). Forexample, the product comprises hydrogen having a water amount of 5 ppmor less (hereinafter referred to as dry hydrogen).

For example, a known dehumidification mechanism for removing watercontained in hydrogen is disclosed in Japanese Laid-Open PatentPublication No. 2004-149890. As shown in FIG. 23, the dehumidificationmechanism contains a dehumidification unit 6. The dehumidification unit6 has a main vessel body 2, a dehumidifying agent 1 for dehumidifyinguntreated gas contained in the main vessel body 2, and a hydrogen gassupply pipe 3 a and a hydrogen gas discharge pipe 3 b connected to lowerand upper ends of the main vessel body 2. The dehumidification unit 6further has a cooling trace 4 for circulating a cooling gas, which iswound helically at approximately regular intervals on the outer surfaceof the main vessel body 2, and a heating wire 5, which is arrangedwithin the cooling trace 4 parallel and adjacent to the cooling trace 4.

In the dehumidification mechanism, hydrogen gas generated byelectrolysis is transferred to the dehumidification unit 6 in adehumidification step, and is introduced into the main vessel body 2through the lower hydrogen gas supply pipe 3 a. The hydrogen gas isdehumidified to a predetermined dew point by the dehumidifying agent 1,and then is discharged from the upper hydrogen gas discharge pipe 3 b tothe outside of the main vessel body 2, and is supplied to a hydrogenstorage unit such as a hydrogen tank.

The recovery process of the dehumidification unit 6 contains the stepsof heating the dehumidifying agent 1 to remove water, and cooling theheated dehumidifying agent 1 to an approximately normal temperature.More specifically, in the heating step, the entire main vessel body 2 isheated by the heating wire 5. In the cooling step, cooling gas isintroduced into the cooling trace 4, whereby the dehumidifying agent 1is cooled to regain a predetermined dehumidification capability.

SUMMARY OF THE INVENTION

However, since the recovery process contains the heating and coolingsteps, the above dehumidification mechanism requires a long recoverytime due to the temperature change. Furthermore, it is necessary toapply a large amount of electricity to the heating wire 5, and thus awater electrolysis apparatus using the dehumidification mechanism ispoor in operational efficiency.

In addition, on initial start up (activation) or after maintenance ofthe water electrolysis apparatus has been performed, water attached tothe hydrogen gas discharge pipe 3 b located downstream of thedehumidification unit 6 is often dropped therefrom. Such dropped wateris introduced into the hydrogen storage unit, so that the hydrogenstored therein exhibits a water amount of more than 5 ppm.

A general object of the present invention is to provide a waterelectrolysis system capable of minimizing energy consumption forhydrogen dehumidification, and which exhibits improved economicefficiency, convenience, and operational efficiency.

Another object of the present invention is to provide a waterelectrolysis system operation method, which even in the event that wateris introduced into a hydrogen storage unit, is capable of easilylowering the water amount in the unit to a threshold value or less.

The present invention relates to a water electrolysis system containinga high-pressure hydrogen production unit for electrolyzing water,thereby generating oxygen at an anode side and generating high-pressurehydrogen at a cathode side, the pressure of the high-pressure hydrogenbeing higher than that of the oxygen, a gas-liquid separation unit forremoving water contained in the high-pressure hydrogen, and which isplaced on a hydrogen pipe for discharging the high-pressure hydrogenfrom the high-pressure hydrogen production unit, and a hydrogen supplypipe for transferring dewatered high-pressure hydrogen from thegas-liquid separation unit.

In the water electrolysis system, a cooling unit for variablycontrolling temperature of the high-pressure hydrogen, thereby adjustingthe humidity of the high-pressure hydrogen, is placed on the hydrogensupply pipe.

In the present invention, since the temperature of the high-pressurehydrogen can be variably controlled in the water electrolysis system,the high-pressure hydrogen can be dehumidified efficiently and reliably.Furthermore, excess energy consumption can be prevented during hydrogendehumidification to thereby improve economic efficiency and convenience.Thus, the overall operational efficiency of the water electrolysissystem can easily be improved using a simple and economical structure.

The present invention further relates to a method for operating a waterelectrolysis system containing a high-pressure hydrogen production unitfor electrolyzing water, thereby generating oxygen at an anode side andgenerating hydrogen at a cathode side, a hydrogen storage unit forstoring hydrogen discharged from the high-pressure hydrogen productionunit, a hydrogen supply pipe for supplying hydrogen generated in thehigh-pressure hydrogen production unit to the hydrogen storage unit, anda water adsorption unit, which is connected to the hydrogen supply pipe,for adsorbing water contained in the hydrogen generated in thehigh-pressure hydrogen production unit.

The operation method includes the steps of starting the waterelectrolysis system, measuring an elapsed time from start of the waterelectrolysis system, and preventing electrolysis in the waterelectrolysis system from stopping if the measured elapsed time is lessthan a set time.

In the present invention, during a predetermined time after initiationof the water electrolysis system, stopping of electrolysis is preventedand the generated hydrogen is supplied to the hydrogen storage unit.Therefore, even if water attached to the hydrogen supply pipe isintroduced into the hydrogen storage unit at the start of the waterelectrolysis system, the dry hydrogen generated by electrolysis can besupplied continuously to the hydrogen storage unit over thepredetermined time, whereby the dew point in the hydrogen storage unitcan be lowered to reliably maintain the water concentration at athreshold value or less.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural view of a water electrolysis systemaccording to a first embodiment of the present invention;

FIG. 2 is a flowchart illustrating operations of the water electrolysissystem of FIG. 1;

FIG. 3 is a characteristic map showing relationships between pressure, acooling water amount, and Peltier power consumption;

FIG. 4 is a characteristic curve showing a relationship between hydrogenpressure and water concentration;

FIG. 5 is a characteristic curve showing a relationship between hydrogentemperature and water concentration;

FIG. 6 is a schematic structural view of a water electrolysis systemaccording to a second embodiment of the present invention;

FIG. 7 is a flowchart illustrating operations of the water electrolysissystem of FIG. 6;

FIG. 8 is a characteristic map showing relationships between anelectrolytic current value, a cooling water amount, and Peltier powerconsumption;

FIG. 9 is a schematic structural view of a water electrolysis systemaccording to a third embodiment of the present invention;

FIG. 10 is a flowchart illustrating operations of the water electrolysissystem of FIG. 9;

FIG. 11 is a schematic structural view of a water electrolysis systemaccording to a fourth embodiment of the present invention;

FIG. 12 is a flowchart illustrating operations of the water electrolysissystem of FIG. 11;

FIG. 13 is a schematic structural view of a water electrolysis systemaccording to a fifth embodiment of the present invention;

FIG. 14 is a flowchart illustrating operations of the water electrolysissystem of FIG. 13;

FIG. 15 is a schematic structural view of a water electrolysis systemaccording to a sixth embodiment of the present invention;

FIG. 16 is a flowchart illustrating operations of the water electrolysissystem of FIG. 15;

FIG. 17 is a schematic structural view of a water electrolysis systemaccording to a seventh embodiment of the present invention;

FIG. 18 is a characteristic curve showing relationships between pressurein a hydrogen tank of the water electrolysis system of FIG. 17, waterconcentration after high-dew point hydrogen introduction, and dilutiontime;

FIG. 19 is a flowchart illustrating operations of the water electrolysissystem of FIG. 17;

FIG. 20 is a schematic structural view of a water electrolysis systemaccording to an eighth embodiment of the present invention;

FIG. 21 is a diagram illustrating an amount of water that is adsorbed ina second adsorption column;

FIG. 22 is a flowchart illustrating operations of the water electrolysissystem of FIG. 20; and

FIG. 23 is a partially sectioned front view of a dehumidification unitaccording to the related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a water electrolysis system 10 according to a firstembodiment of the present invention includes a high-pressure hydrogenproduction unit (differential pressure-type water electrolysisapparatus) 12 for electrolyzing water (pure water), thereby producingoxygen (at ordinary pressure) and high-pressure hydrogen (at a pressurehigher than the oxygen pressure, e.g., 1 to 70 MPa). The waterelectrolysis system 10 further includes a water storage unit 14 forseparating oxygen and residual water that is discharged from thehigh-pressure hydrogen production unit 12, and storing the water, awater circulation unit 16 for circulating the water stored in the waterstorage unit 14 through the high-pressure hydrogen production unit 12,and a water supply unit 18 for supplying pure water prepared from citywater to the water storage unit 14.

The water electrolysis system 10 further includes a gas-liquidseparation unit 22 for removing water contained in high-pressurehydrogen, which is discharged from the high-pressure hydrogen productionunit 12 through a hydrogen pipe 20, and a high-pressure hydrogen supplypipe 24 for transferring dewatered high-pressure hydrogen from thegas-liquid separation unit 22. In addition, the water electrolysissystem 10 includes a cooling unit 26, which is disposed on thehigh-pressure hydrogen supply pipe 24, for variably controllingtemperature of the high-pressure hydrogen and thereby adjusting thehumidity of the high-pressure hydrogen, together with a control unit(ECU) 28 for controlling the water electrolysis system 10 in itsentirety.

The high-pressure hydrogen production unit 12 contains a cell stackprepared by stacking a plurality of unit cells 30. At onestacking-direction end of the unit cells 30, a terminal plate 32 a, aninsulation plate 34 a, and an end plate 36 a are disposed in this orderin an outward direction. Similarly, at the other stacking-direction endof the unit cells 30, a terminal plate 32 b, an insulation plate 34 b,and an end plate 36 b are disposed in this order in an outwarddirection. The unit cells 30, the terminal plates 32 a and 32 b, theinsulation plates 34 a and 34 b, and the end plates 36 a and 36 b areintegrally fastened and fixed together in the stacking direction.

Terminals 38 a and 38 b protrude outwardly from side surfaces of theterminal plates 32 a and 32 b respectively. The terminals 38 a and 38 bare electrically connected by wirings 39 a and 39 b to an electrolysispower source 40.

For example, the unit cell 30 contains a disk shaped membrane-electrodeassembly 42, and further contains an anode-side separator 44 and acathode-side separator 46 sandwiching the membrane-electrode assembly 42therebetween. The shapes of the membrane-electrode assembly 42, theanode-side separator 44, and the cathode-side separator 46 are notlimited to disk shapes, but may be selected from various shapes such asrectangular and square shapes.

For example, the membrane-electrode assembly 42 contains a solid polymerelectrolyte membrane 48 prepared by impregnating a thinperfluorosulfonic acid membrane with water, and further contains ananode-side current collector 50 and a cathode-side current collector 52disposed on either side of the solid polymer electrolyte membrane 48.

An anode catalyst layer 50 a and a cathode catalyst layer 52 a areformed on either side of the solid polymer electrolyte membrane 48. Forexample, the anode catalyst layer 50 a contains a Ru (ruthenium)catalyst, and the cathode catalyst layer 52 a contains a platinumcatalyst.

A water supply through hole 56 for supplying water (pure water), adischarge through hole 58 for discharging oxygen generated by thereaction and the unreacted water, and a hydrogen through hole 60 fortransferring hydrogen generated by the reaction are formed and extend inthe stacking direction on the periphery of the unit cells 30.

A first flow path 64 connected to the water supply through hole 56 andthe discharge through hole 58 is formed on the surface of the anode-sideseparator 44 facing the membrane-electrode assembly 42. The first flowpath 64 is formed within a surface area of the anode-side currentcollector 50, and contains a plurality of flow channels and embossedportions, etc. The oxygen generated by the reaction and the unreactedwater are transferred in the first flow path 64.

A second flow path 68, which is connected to the hydrogen through hole60, is formed on the surface of the cathode-side separator 46 facing themembrane-electrode assembly 42. The second flow path 68 is formed withina surface area of the cathode-side current collector 52, and contains aplurality of flow channels and embossed portions, etc. The high-pressurehydrogen generated by the reaction is transferred to the second flowpath 68.

The water circulation unit 16 has a circulation pipe 72 connected to thewater supply through hole 56 in the high-pressure hydrogen productionunit 12. A circulation pump 74 and an ion exchanger 76 are placed on thecirculation pipe 72, and the end of the circulation pipe 72 is connectedto the bottom of a tank 78 in the water storage unit 14.

One end of a return pipe 80 is connected to the top of the tank 78, andthe other end thereof is connected to the discharge through hole 58 inthe high-pressure hydrogen production unit 12. The one end of the returnpipe 80 is positioned such that the one end is opened constantly andlocated in the water stored in the tank 78.

The tank 78 is connected to a pure water supply pipe 84, which extendsfrom the water supply unit 18 to an oxygen discharge pipe 86, fordischarging oxygen that is separated from the pure water in the tank 78.

One end of the hydrogen pipe 20 is connected to the hydrogen throughhole 60 in the high-pressure hydrogen production unit 12, and the otherend thereof is connected to the gas-liquid separation unit 22. Thegas-liquid separation unit 22 has a tank 88 for storing water (WS). Adrain line 90 is connected to the bottom of the tank 88, and a drainvalve 92 is formed in the drain line 90.

High-pressure hydrogen is dewatered by the gas-liquid separation unit22. The obtained dry hydrogen is introduced into the high-pressurehydrogen supply pipe 24. The cooling unit 26, which is placed on thehigh-pressure hydrogen supply pipe 24, contains a Peltier dehumidifier(Peltier element) 94 and a heat exchanger 96.

The Peltier dehumidifier 94 utilizes a Peltier element cooler andcontains a variable source 97. A coolant pipe 98 is connected to thePeltier dehumidifier 94 in order to release heat from thehigh-temperature side. Instead of the coolant pipe 98, a radiation finor the like may be used.

The heat exchanger 96 is located in series on an upstream side of thePeltier dehumidifier 94. The heat exchanger 96 is connected to a coolingwater supply pipe 100 for supplying cooling water as a coolant, and acooling water discharge pipe 102 for discharging the cooling water. Aflow control valve 104 for variably controlling the flow rate of thecooling water introduced into the heat exchanger 96 is disposed on thecooling water supply pipe 100.

The cooling water discharge pipe 102 may be connected to the inlet sideof the coolant pipe 98, such that water for electrolysis is circulatedthrough the cooling water supply pipe 100. In this case, another watersupply source is not required, whereby the overall structure of thewater electrolysis system 10 can be simplified. Alternatively, thecooling water discharge pipe 102 may be provided separately from thecoolant pipe 98, such that water for electrolysis is supplied to onlyone of the heat exchanger 96 and the Peltier dehumidifier 94.

The control unit 28 contains a pressure detection means (pressuredetector) 110 for detecting the cathode-side pressure of thehigh-pressure hydrogen production unit 12, and a current adjustmentmeans (current adjuster) 112 for variably controlling a current appliedto the Peltier dehumidifier 94 based on the detected pressure. Inaddition, the control unit 28 further contains a cooling water amountadjustment means (coolant amount adjuster) 114 for variably controllingthe amount of cooling water (coolant) introduced into the heat exchanger96 based on the detected pressure.

A pressure sensor 116 for measuring the cathode-side pressure of thehigh-pressure hydrogen production unit 12 is placed on the hydrogen pipe20. Measurement signals are transferred from the pressure sensor 116 tothe pressure detection means 110.

A condenser 118 and a back pressure valve 120 are located on thehigh-pressure hydrogen supply pipe 24 on the downstream side of thecooling unit 26. For example, the condenser 118 contains a sinteredfilter or the like. The back pressure valve 120 is opened at apredetermined set pressure (e.g., 35 MPa) in order to supplyhigh-pressure hydrogen as a hydrogen product to a fuel cell vehicle (notshown) or the like.

For example, in the control unit 28 of the water electrolysis system 10,a map of FIG. 3 is prepared from the relationships between thecathode-side pressure P of the high-pressure hydrogen production unit12, the Peltier power consumption W of the Peltier dehumidifier 94, andthe cooling water amount Q of the heat exchanger 96, which is controlledbased on the pressure P. A map is prepared based on the relationshipbetween the hydrogen pressure and the water concentration shown in FIG.4, as well as the relationship between the hydrogen temperature and thewater concentration shown in FIG. 5.

More specifically, as shown in FIG. 4, the water concentration of thehydrogen decreases as the hydrogen pressure (cathode-side pressure P)increases (i.e., becomes higher). When the water concentration isdecreased, the flow rate of the hydrogen is lowered. Therefore, hydrogencan be dehumidified more easily, and the Peltier power consumption W ofthe Peltier dehumidifier 94 required for dehumidification can bereduced.

Furthermore, as shown in FIG. 5, under a low hydrogen pressure(represented by the dashed line), in order to decrease the waterconcentration, the cooling water amount Q of the heat exchanger 96 mustbe increased in order to lower the hydrogen temperature. In contrast,under a high hydrogen pressure (represented by the continuous line), thewater concentration can be decreased even at a higher hydrogentemperature, and thus the required cooling water amount Q can bereduced.

Operations of the water electrolysis system 10 having the structureshown in FIG. 1 will be described below with reference to the flowchartof FIG. 2.

When electrolysis is started in the water electrolysis system 10 (stepS1), pure water prepared from city water is supplied from the watersupply unit 18 to the tank 78 in the water storage unit 14.

The water in the tank 78 is supplied by the circulation pump 74 in thewater circulation unit 16 through the circulation pipe 72 to the watersupply through hole 56 in the high-pressure hydrogen production unit 12.Meanwhile, a voltage (an electrolytic current value A) is applied to theterminals 38 a and 38 b on the terminal plates 32 a and 32 b by theelectrolysis power source 40, which is electrically connected thereto(step S2).

In each unit cell 30, water is supplied from the water supply throughhole 56 to the first flow path 64 on the anode-side separator 44, andthe water is transferred along the anode-side current collector 50.Thus, water is electrically decomposed on the anode catalyst layer 50 ato generate hydrogen ions, electrons, and oxygen. Hydrogen ionsgenerated by the positive electrode reaction are transferred through thesolid polymer electrolyte membrane to the cathode catalyst layer 52 a,and become bonded with electrons to produce hydrogen.

The produced hydrogen flows through the second flow path 68 between thecathode-side separator 46 and the cathode-side current collector 52. Thehydrogen has a pressure, which is higher than that of the water in thewater supply through hole 56, and thus the hydrogen can be transferredthrough the hydrogen through hole 60 and discharged to the outside ofthe high-pressure hydrogen production unit 12.

The oxygen generated in the reaction and unreacted water flow throughthe first flow path 64, and a fluid mixture thereof is discharged fromthe discharge through hole 58 to the return pipe 80 in the watercirculation unit 16. The oxygen and the unreacted water are introducedto the tank 78 and separated therein. The separated water is introducedby the circulation pump 74 through the circulation pipe 72 and the ionexchanger 76, and into the water supply through hole 56. The separatedoxygen is discharged to the outside through the oxygen discharge pipe86.

Hydrogen generated in the high-pressure hydrogen production unit 12 istransferred through the hydrogen pipe 20 to the gas-liquid separationunit 22. In the gas-liquid separation unit 22, gaseous or liquid water(moisture) contamed in the hydrogen is separated from the hydrogen andstored in the tank 88. The resultant hydrogen is introduced into thehigh-pressure hydrogen supply pipe 24.

As described above, as water electrolysis and hydrogen productioncontinue to be carried out in the high-pressure hydrogen production unit12, the cathode-side pressure (hydrogen pressure) P is increased to theset pressure of the back pressure valve 120. The cathode-side pressure Pof the high-pressure hydrogen production unit 12 is detected by thepressure detection means 110 in the control unit 28 based on themeasurement signal from the pressure sensor 116 (step S3). Based on thedetected pressure P, in the pressure detection means 110, a coolingwater amount Q_(map) of the cooling water supplied to the heat exchanger96, and a Peltier power consumption W_(map) of the current applied tothe Peltier dehumidifier 94 are calculated and read out respectively,based on the relationships between the pressure P, the cooling wateramount Q, and the Peltier power consumption W, as shown in the map ofFIG. 3.

Based on the cooling water amount Q_(map) read from the map of FIG. 3,the flow control valve 104 is controlled by the cooling water amountadjustment means 114 in order to adjust the amount Q of cooling watersupplied from the cooling water supply pipe 100 to the heat exchanger 96(step S4). Thus, heat exchange is performed between the cooling waterand high-pressure hydrogen introduced through the high-pressure hydrogensupply pipe 24 into the heat exchanger 96, whereby the hydrogen iscooled to a temperature at which the hydrogen can be dehumidified to adesired water concentration (see FIG. 5).

High-pressure hydrogen is transferred from the heat exchanger 96 to thePeltier dehumidifier 94. Based on the Peltier power consumption W_(map)read from the map of FIG. 3, the current (Peltier power consumption W)applied from the variable source 97 to the Peltier dehumidifier 94 iscontrolled by the current adjustment means 112 (step S4). Thus,high-pressure hydrogen is dehumidified to a desired humidity by thePeltier dehumidifier 94, whereby dry hydrogen is obtained.

Dry hydrogen is transferred from the Peltier dehumidifier 94 through thecondenser 118 and to the back pressure valve 120. When the pressure ofthe dry hydrogen is increased to a set pressure, the back pressure valve120 is opened to supply hydrogen as a hydrogen product to a fuel cellvehicle (not shown) or the like.

Then, the high-pressure hydrogen production unit is operated steadily(step S5), and electrolysis is stopped (step S6) in order to bring theoperation of the water electrolysis system 10 to an end.

As described above, in the first embodiment, the cooling unit 26, whichis capable of variably controlling temperature of the high-pressurehydrogen to adjust the humidity thereof, is disposed on thehigh-pressure hydrogen supply pipe 24. Therefore, even when the pressureof the high-pressure hydrogen increases or decreases due to any ofvarious factors, the high-pressure hydrogen can be cooled sufficientlyin the high-pressure hydrogen supply pipe 24 depending on the detectedpressure. Thus, hydrogen is not cooled excessively by the cooling unit26, and the overall operational efficiency of the water electrolysissystem 10 can be improved.

Furthermore, in the water electrolysis system 10, power consumption ofthe Peltier dehumidifier 94 for dehumidifying hydrogen, and the amount Qof cooling water supplied to the heat exchanger 96 can be reduced tothereby curb energy consumption, and thus, economic efficiency andconvenience can be improved. Thus, advantageously, the overalloperational efficiency of the water electrolysis system 10 can beimproved easily using a simple and economical structure.

The cooling unit 26 contains the Peltier dehumidifier 94 and the heatexchanger 96 located upstream of the Peltier dehumidifier 94. Therefore,the amount Q of cooling water and the Peltier power consumption W of thePeltier dehumidifier 94 can be variably controlled, respectively,depending on the detected cathode-side pressure P of the high-pressurehydrogen production unit 12, and based on relationships between thepressure P, the cooling water amount Q, and the Peltier powerconsumption W (the map of FIG. 3).

Thus, high-pressure hydrogen can be dehumidified to a desired waterconcentration using a minimum amount Q of cooling water and minimumPeltier power consumption W, whereby advantageously, the operationalefficiency of the water electrolysis system 10 can be significantlyimproved.

In addition, since the heat exchanger 96 is located upstream of thePeltier dehumidifier 94, high-pressure hydrogen is transferred to thePeltier dehumidifier 94 after water contained in the hydrogen primarilyis removed by the heat exchanger 96. Therefore, advantageously, powerconsumption (Peltier power consumption W) of the Peltier dehumidifier 94can be reduced.

A water electrolysis system 130 according to a second embodiment of thepresent invention is shown in FIG. 6. The same components are markedusing the same reference numerals in the water electrolysis system 130of the second embodiment and the water electrolysis system 10 of thefirst embodiment, and detailed explanations of such features are omittedin the second embodiment. Detailed explanations of such features alsoare omitted in the third to eighth embodiments, to be describedhereinafter.

The water electrolysis system 130 includes a control unit 132, whichcorresponds to the control unit 28 of the first embodiment. The controlunit 132 contains a current detection means (current detector) 134 fordetecting an electrolytic current value A of the high-pressure hydrogenproduction unit 12. In the high-pressure hydrogen production unit 12, acurrent detection sensor 136 for detecting the electrolytic currentvalue A is disposed on the electrolysis power source 40.

For example, in the control unit 132 of the water electrolysis system130, at a constant cathode-side pressure P of the high-pressure hydrogenproduction unit 12, the map shown in FIG. 8 is prepared fromrelationships between the electrolytic current value A of theelectrolysis power source 40 and the cooling water amount Q of the heatexchanger 96. The Peltier power consumption W of the Peltierdehumidifier 94 is controlled based on the current value A.

The amount of produced hydrogen can be changed by controlling theelectrolytic current value A. For example, as shown in the map of FIG.8, when the electrolytic current value A is increased, the cooling wateramount Q and the Peltier power consumption W are increased in order toefficiently cool and dehumidify the increased hydrogen amount.

Operations of the water electrolysis system 130 having the abovestructure will be described below with reference to the flowchart ofFIG. 7. Steps S11, S12, S16, and S17 are the same, respectively, assteps S1, S2, S5, and S6 of the first embodiment (see FIG. 2).

Water electrolysis is started in the water electrolysis system 130 (stepS11) and is carried out at an electrolytic current value A in thehigh-pressure hydrogen production unit 12 (step S12). The cathode-sidepressure P of the high-pressure hydrogen production unit 12 is detectedby the pressure detection means 110. When the detected pressure P isjudged to have become constant (YES in step S13), the next step S14 iscarried out.

In step S14, the electrolytic current value A is detected by the currentdetection means 134 using the current detection sensor 136 disposed onthe electrolysis power source 40. A cooling water amount Q_(map) of theheat exchanger 96 and a Peltier power consumption W_(map) of the Peltierdehumidifier 94, which correspond to the detected electrolytic currentvalue A, are calculated or read out respectively from the relationshipsbetween the electrolytic current value A, the cooling water amount Q,and the Peltier power consumption W, as shown in the map of FIG. 8.Based on the cooling water amount Q_(map) and the Peltier powerconsumption W_(map), the amount Q of the cooling water supplied to theheat exchanger 96, and the current value applied from the variablesource 97 to the Peltier dehumidifier 94 are controlled (step S15).Then, the water electrolysis system 130 is operated steadily (step S16),and thereafter, electrolysis is stopped (step S17).

Consequently, in the second embodiment, when the pressure P of thehigh-pressure hydrogen in the hydrogen pipe 20 is maintained at aconstant pressure, although the hydrogen production amount changesdepending on the electrolytic current value A of the electrolysis powersource 40, hydrogen can be dehumidified using a minimum cooling wateramount Q and minimum Peltier power consumption W. Therefore, powerconsumption of the Peltier dehumidifier 94 for dehumidifying hydrogen,and the amount Q of cooling water supplied to the heat exchanger 96 canbe reduced in order to curb energy consumption, and advantageously, thesame effects as those of the first embodiment can be achieved as well inthe second embodiment. For example, in the second embodiment as well,overall operational efficiency of the water electrolysis system 130 canbe improved significantly.

A water electrolysis system 150 according to a third embodiment of thepresent invention is shown in FIG. 9.

The water electrolysis system 150 has a cooling unit 152 and a controlunit 154, which correspond to the cooling unit 26 and the control unit28 of the first embodiment. Similar to the first embodiment, the coolingunit 152 contains the Peltier dehumidifier 94, and the control unit 154contains the pressure detection means 110 and the current adjustmentmeans 112, however, the water electrolysis system 150 does not have theheat exchanger 96 or the cooling water amount adjustment means 114.

The water electrolysis system 150 is operated in accordance with theflowchart of FIG. 10, which contains steps S21 to S26. Steps S21 to S23,S25, and S26 are the same as steps S1 to S3, S5, and S6 of the firstembodiment (see FIG. 2), respectively, and detailed explanations of suchsteps are omitted in the third embodiment.

After steps S21 and S22, in step S23, the pressure P is detected by thepressure detection means 110. Based on the detected pressure P, thePeltier power consumption W_(map) is read out from the relationshipbetween the pressure P and the Peltier power consumption W, as shown inFIG. 3. Based on the Peltier power consumption W_(map), the currentvalue applied to the Peltier dehumidifier 94 is controlled by thecurrent adjustment means 112 (step S24). Thereafter, steps S25 and S26are carried out.

Consequently, in the water electrolysis system 150 according to thethird embodiment, electricity is not applied excessively to the Peltierdehumidifier 94. Therefore, advantageously, the same effects as those ofthe first and second embodiments can also be achieved in the thirdembodiment. For example, energy consumption required for hydrogendehumidification can be minimized as well in the third embodiment.

A water electrolysis system 160 according to a fourth embodiment of thepresent invention is shown in FIG. 11.

The water electrolysis system 160 has a cooling unit 162 and a controlunit 164, which correspond to the cooling unit 26 and the control unit28 of the first embodiment. The cooling unit 162 contains the heatexchanger 96. The control unit 164 contains the pressure detection means110 and the cooling water amount adjustment means 114. The waterelectrolysis system 160 does not have the Peltier dehumidifier 94 or thecurrent adjustment means 112.

The water electrolysis system 160 is operated in accordance with theflowchart of FIG. 12, which contains steps S31 to S36. Steps S31 to S33,S35, and S36 are the same as steps S1 to S3, S5, and S6 of the firstembodiment (see FIG. 2), respectively, and detailed explanations of suchsteps are omitted in the fourth embodiment.

After steps S31 and S32, the pressure P is detected by the pressuredetection means 110 in step S33. Based on the detected pressure P, thecooling water amount Q_(map) is read out from the relationship betweenthe pressure P and the cooling water amount Q, as shown in FIG. 3. Basedon the cooling water amount Q_(map), the amount Q of the cooling watersupplied to the heat exchanger 96 is controlled by the cooling wateramount adjustment means 114 (step S34). Thereafter, steps S35 and S36are carried out.

Consequently, according to the fourth embodiment, the amount Q ofcooling water supplied to the heat exchanger 96 in the waterelectrolysis system 160 is reduced. Therefore, the same advantageouseffects as those of the first and second embodiments can be achieved inthe fourth embodiment as well. For example, energy consumption requiredfor hydrogen dehumidification can also be minimized in the fourthembodiment.

A water electrolysis system 170 according to a fifth embodiment of thepresent invention is shown in FIG. 13.

The water electrolysis system 170 has a cooling unit 172 and a controlunit 174, which correspond to the cooling unit 26 and the control unit132 of the second embodiment. The cooling unit 172 contains the Peltierdehumidifier 94, and the control unit 174 contains the pressuredetection means 110, the current adjustment means 112, and the currentdetection means 134. The water electrolysis system 170 does not includethe heat exchanger 96 or the cooling water amount adjustment means 114.

The water electrolysis system 170 is operated in accordance with theflowchart of FIG. 14, which contains steps S41 to S47. Steps S41 to S44,S46, and S47 are the same as steps S11 to S14, S16, and S17 of thesecond embodiment (see FIG. 7), respectively, and detailed explanationsof such steps are omitted in the fifth embodiment.

After steps S41 to S43, in step S44, an electrolytic current value A isdetected by the current detection means 134. Based on the detectedelectrolytic current value A, the Peltier power consumption W_(map) isread out from the relationship between the electrolytic current value Aand the Peltier power consumption W, as shown in FIG. 8. Based on thePeltier power consumption W_(map), the current value applied to thePeltier dehumidifier 94 is controlled by the current adjustment means112 (step S45). Thereafter, steps S46 and S47 are carried out.

Consequently, according to the fifth embodiment, electricity is notapplied excessively to the Peltier dehumidifier 94 in the waterelectrolysis system 170. Therefore, the same advantageous effects asthose of the first to third embodiments can also be achieved in thefifth embodiment. For example, energy consumption required for hydrogendehumidification can also be minimized in the fifth embodiment.

A water electrolysis system 180 according to a sixth embodiment of thepresent invention is shown in FIG. 15.

The water electrolysis system 180 has a cooling unit 182 and a controlunit 184, which correspond to the cooling unit 26 and the control unit132 of the second embodiment. The cooling unit 182 contains the heatexchanger 96, and the control unit 184 contains the pressure detectionmeans 110, the cooling water amount adjustment means 114, and thecurrent detection means 134. The water electrolysis system 180 does notinclude the Peltier dehumidifier 94 or the current adjustment means 112.

The water electrolysis system 180 is operated in accordance with theflowchart of FIG. 16, which contains steps S51 to S57. Steps S51 to S54,S56, and S57 are the same as steps S11 to S14, S16, and S17 of thesecond embodiment (see FIG. 7), respectively, and detailed explanationsof such steps are omitted in the sixth embodiment.

After steps S51 to S53, in step S54, an electrolytic current value A isdetected by the current detection means 134. Based on the detectedelectrolytic current value A, the cooling water amount Q_(map) is readout from the relationship between the electrolytic current value A andthe cooling water amount Q, as shown in FIG. 8. Based on the coolingwater amount Q_(map), the amount Q of cooling water supplied to the heatexchanger 96 is controlled by the cooling water amount adjustment means114 (step S55). Thereafter, steps S56 and S57 are carried out.

Consequently, in the sixth embodiment, the amount Q of cooling watersupplied to the heat exchanger 96 in the water electrolysis system 180is reduced. Therefore, the same advantageous effects as those of thefirst, second, and fourth embodiments can be achieved in the sixthembodiment as well. For example, energy consumption required forhydrogen dehumidification can also be minimized in the sixth embodiment.

In order to obtain dry hydrogen more reliably, an adsorber may be placedon a downstream side of the Peltier dehumidifier 94. For example, thewater electrolysis system 10, 130, 150, 170 may contain an adsorber thatuses a replaceable adsorbent. In this case, as compared with aconventional system, which uses only an adsorber for dehumidificationwithout the Peltier dehumidifier 94, the adsorbent replacement frequencycan be significantly reduced in the water electrolysis system 10, 130,150, 170, because most of the water can be removed by the Peltierdehumidifier 94. In addition, the water electrolysis system 10, 130,150, 170 can be reduced in size compared to a conventional system.

A water electrolysis system 200 according to a seventh embodiment of thepresent invention is shown in FIG. 17. The water electrolysis system 200includes a hydrogen supply pipe 202 (202 a), which corresponds to thehigh-pressure hydrogen supply pipe 24 of the first embodiment, andfurther has a hydrogen tank (hydrogen storage unit) 204 for storinghigh-pressure hydrogen that is discharged from the high-pressurehydrogen production unit 12. In addition, the water electrolysis system200 has an adsorption column (water adsorption unit) 206 for adsorbingwater contained in the high-pressure hydrogen generated in thehigh-pressure hydrogen production unit 12, and which is placed on thehydrogen supply pipe 202 (202 a). Further, the water electrolysis system200 has a control unit (ECU) 208 for controlling the water electrolysissystem 200 in its entirety.

In the water electrolysis system 200, a hydrogen production unit forgenerating ordinary-pressure hydrogen (i.e., for generating hydrogen andoxygen at the same pressure) may be used instead of the high-pressurehydrogen production unit 12.

The adsorption column 206 and the hydrogen tank 204 are connected inseries on the hydrogen supply pipe 202 (202 a). High-pressure hydrogen,which is stored in the hydrogen tank 204, can be supplied as a hydrogenproduct to a fuel cell vehicle (not shown) or the like. A dehumidifyingagent, such as an adsorbent (not shown) for removing water contained inthe hydrogen, is placed in the adsorption column 206.

A dew point meter (water amount detection unit, DP) 210 and a backpressure valve 212 are arranged in the hydrogen flow direction betweenthe adsorption column 206 and the hydrogen tank 204 on the hydrogensupply pipe 202 (202 a). The dew point meter 210 is used for judgingwhether or not breakthrough of the adsorption column 206 has occurred.Breakthrough implies that the adsorbent has reached the water adsorptionsaturation point, wherein water that should be removed instead leaksfrom the adsorption column 206.

The control unit 208 contains a timer 214 for measuring the elapsed timefrom initiation of the water electrolysis system 200, and furthercontains an operation stop judgment unit (operation stop judgmentdevice) 216 for stopping operation of the water electrolysis system 200when the water amount detected by the dew point meter 210 is greaterthan a threshold amount.

Operations of the water electrolysis system 200 having theaforementioned structure will be described below.

When electrolysis is started in the water electrolysis system 200 andsteady operation (hydrogen production) is started in the high-pressurehydrogen production unit 12, hydrogen is generated in the high-pressurehydrogen production unit 12 and is transferred to the hydrogen supplypipe 202, in the same manner as in steps S2 and S3 according to thefirst embodiment (see FIG. 2). It should be noted that, during thesesteps, the back pressure valve 212 remains closed.

Hydrogen in the hydrogen supply pipe 202 is introduced into theadsorption column 206, and water contained in the hydrogen is adsorbedinto the adsorbent, whereby the water is removed. When the outlet-sidepressure of the adsorption column 206 is increased to the set pressureof the back pressure valve 212, the back pressure valve 212 is opened inorder to introduce hydrogen into the hydrogen tank 204. Hydrogen in thehydrogen tank 204 is supplied as a fuel gas to a fuel cell vehicle (notshown), for example.

A method of operating the water electrolysis system 200 according to theseventh embodiment will be described below.

The operation method essentially includes the steps of starting thewater electrolysis system 200, measuring the elapsed time frominitiation of the water electrolysis system 200, and preventing stoppingof electrolysis in the water electrolysis system 200 if the measuredelapsed time is less than a set time.

The set elapsed time from starting of the water electrolysis system 200,for which stopping of electrolysis is prevented, is selected beforehandbased on the water concentration in the hydrogen tank 204. Morespecifically, before starting the water electrolysis system 200, whenhydrogen in the hydrogen supply pipe 202 a between the outlet of theadsorption column 206 and the inlet of the back pressure valve 212(forming part of the hydrogen supply pipe 202) has a pressure P′ (e.g.,35 MPa), a volume V′, a compression factor Z′, a temperature T′, a gasconstant R′, and a water concentration C′_(H2O), a molar number n′ isobtained using the equation n′=P′·V′/Z′·R′·T′, and a standard-statevolume V′_(std) is obtained using the equationV′_(std)=(P′/P_(std))×(T_(std)/T′)×(V′/Z′). In the foregoing equation,P_(std) is the standard-state pressure, and T_(std) is thestandard-state temperature. Such symbols retain the same meanings in allof the following descriptions.

Meanwhile, before starting the water electrolysis system 200, when thehydrogen in the hydrogen tank 204 has a pressure P₁, a volume V, acompression factor Z, a temperature T, a gas constant R, and a waterconcentration C_(H2O), the molar number n is obtained using the equationn=P₁·V/Z·R·T, and the standard-state volume V_(std) is obtained usingthe equation V_(std)=(P₁/P_(std))×(T_(std)/T)×(V/Z).

After the hydrogen supply pipe 202 a has been filled with hydrogen, thewater concentration C_(H2O) _(—) _(tank) in the hydrogen tank 204 isobtained using the equation C_(H2O) _(—)_(tank)=(n·C_(H2O)+n′·C′_(H2O))/(n+n′), and the standard-state volumeV_(tank) _(—) _(std) of the hydrogen in the hydrogen tank 204 isobtained using the equation V_(tank) _(—) _(std)=V_(std)+V′_(std).Furthermore, when the hydrogen product has a flow rate F and a waterconcentration C_(H2O) we, the time (set time) required to reduce thewater concentration C_(H2O) tank in the hydrogen tank 204 to apredetermined threshold value (e.g., 5 ppm) is obtained using theequation t_(a)={(C_(H2O) _(—) _(tank)−5)×V_(tank) _(—)_(std)}/{(5−C_(H2O) _(—) _(we))×F}.

The pressure P₁ in the hydrogen tank 204 and the time t_(a) required toreduce the water concentration to 5 ppm in the hydrogen tank 204 followthe relationship shown in FIG. 18. Thus, the amount of water introducedinto the hydrogen tank 204, and the time t_(a) required to reduce thewater concentration to 5 ppm remain constant regardless of the pressureP₁ in the hydrogen tank 204.

Operations of the water electrolysis system 200 using theabove-described set time will be described below with reference to theflowchart of FIG. 19.

When the water electrolysis system 200 is started (step S61), thecontrol unit 208 acts to prevent stopping of electrolysis in the waterelectrolysis system 200 (step S62), and measurement of elapsed timeperformed by the timer 214 is started (step S63).

When the elapsed time measured by the timer 214 is judged to havereached the predetermined set time (the time t_(a) required to dilutethe water concentration to 5 ppm) (YES in step S64), detection of theamount of water contained in the hydrogen that is transferred to thehydrogen tank 204 is started in step S65. More specifically, the amountof water in the hydrogen that is transferred from the adsorption column206 to the hydrogen tank 204 is detected by the dew point meter 210,which is placed on the hydrogen supply pipe 202 a between the adsorptioncolumn 206 and the hydrogen tank 204.

Then, in step S66, a judgment is made in the operation stop judgmentunit 216 as to whether or not the water electrolysis system 200 can becontinuously operated. If the water amount detected by the dew pointmeter 210 is larger than the predetermined threshold amount (e.g., 5ppm) (YES in step S66), then the operation stop judgment unit 216 actsto stop operation of the water electrolysis system 200, and a warningmessage is displayed if necessary (step S67).

When the water electrolysis system 200 is stopped, water frequentlybecomes attached to the inner side of the hydrogen supply pipe 202 (202a). Water that becomes attached to the hydrogen supply pipe 202 (202 a)is likely to enter into the hydrogen tank 204 when the waterelectrolysis system 200 is started. Therefore, in the event thatelectrolysis is stopped immediately after initiation of the waterelectrolysis system 200, the water concentration in the hydrogen tank204 may be higher than the threshold value (e.g., 5 ppm).

As described above, according to the seventh embodiment, the timerequired for lowering the dew point in the hydrogen tank 204, so as toreduce the water concentration to the threshold value (e.g., 5 ppm) orless, is used as the set time t_(a), and stopping of electrolysis in thewater electrolysis system 200 is prevented within a set time afterinitiation of the water electrolysis system 200. Hydrogen generated inthe high-pressure hydrogen production unit 12 is supplied from thegas-liquid separation unit 22, through the adsorption column 206, and tothe hydrogen tank 204. Therefore, advantageously, water concentration isreliably maintained at the threshold value or less in the hydrogen tank204.

A water electrolysis system 220 according to an eighth embodiment of thepresent invention is shown in FIG. 20. The same components are markedwith the same reference numerals as in the water electrolysis system 220of the eighth embodiment and the water electrolysis system 200 of theseventh embodiment, and detailed explanations of such features areomitted in the eighth embodiment.

The water electrolysis system 220 has a hydrogen supply pipe 222 fortransferring hydrogen from the gas-liquid separation unit 22. A firstadsorption column 206 a, a second adsorption column 206 b, and thehydrogen tank 204 are arranged on the hydrogen supply pipe 222 along thehydrogen flow direction.

A first dew point meter (first DP) 210 a is connected to a hydrogensupply pipe 222 a between the first adsorption column 206 a and thesecond adsorption column 206 b. A second dew point meter (second DP) 210b and a back pressure valve 212 are connected respectively to a hydrogensupply pipe 222 b between the second adsorption column 206 b and thehydrogen tank 204. The respective hydrogen supply pipes 222 a and 222 bmake up parts of the hydrogen supply pipe 222.

The first dew point meter 210 a is used for judging whether or notbreakthrough of the first adsorption column 206 a has occurred. On theother hand, the second dew point meter 210 b is used for judging whetheror not breakthrough of the second adsorption column 206 b has occurred.

In the eighth embodiment, immediately after starting electrolysis in thewater electrolysis system 220, electrolysis is carried out for apredetermined time without regard to the water amount (dew point)detected by the first dew point meter 210 a. Then, a judgment is made asto whether or not breakthrough of the first adsorption column 206 a hasoccurred based on a value detected by the first dew point meter 210 a.The predetermined time may be selected depending on the water-handlingcapacity of the second adsorption column 206 b, etc.

More specifically, as shown in FIG. 21, the total water adsorptioncapacity W_(all) of the second adsorption column 206 b is calculated.When breakthrough of the first adsorption column 206 a occurs, an amountW_(a) of water is transferred into the second adsorption column 206 bduring one operational cycle of the water electrolysis system 220.Further, the water electrolysis system 220 is operated at most k timesfor a given an interval of time in order to replace the first adsorptioncolumn 206 a. In this case, the sum of the amount of water transferredto the second adsorption column 206 b during the time interval requiredfor replacing the first adsorption column 206 a is obtained by theproduct W_(a)×k.

When breakthrough of the first adsorption column 206 a occurs, thesecond adsorption column 206 b has a backup capacity W_(b) (an amount ofwater transferred during a time required for replacement). In this case,the predetermined time is selected in view of satisfying the inequalityW_(all)>W_(a)×k+W_(b).

More specifically, the amount W_(min) of water absorbed in the secondadsorption column 206 b during the predetermined time (e.g., 1 minute)is obtained using the equation W_(min)=L×W₁, where L is a discharge flowamount and W₁ is a water amount at a predetermined dew point. Inaddition, the time t_(dry) (min), at which hydrogen having thepredetermined dew point can be dried by the second adsorption column 206b, is obtained using the equation t_(dry)=W_(all)/(L×W₁).

When operation of the water electrolysis system 220 continues for agiven time t₁ after breakthrough of the first adsorption column 206 a,the predetermined time t_(b), at which the dew point of the firstadsorption column 206 a is not detected by the first dew point meter 210a immediately after start of electrolysis, is obtained using theequation t_(b)=(W_(all)/L·W₁−t₁)/k.

A method for operating the water electrolysis system 220 according tothe eighth embodiment will be described below with reference to theflowchart of FIG. 22.

After the water electrolysis system 220 has been started (step S71), instep S72, it is judged whether or not an instruction to replace thefirst adsorption column 206 a has been provided. If the instruction toreplace the first adsorption column 206 a is not provided (YES in stepS72), then step S73 is carried out. Electrolysis is started, whereuponhydrogen is transferred through the first adsorption column 206 a. Whena predetermined time t_(b) has elapsed from starting of the waterelectrolysis system 220, a judgment is made as to whether or notbreakthrough of the first adsorption column 206 a has occurred based ona detection signal from the first dew point meter 210 a.

Normal operations are carried out using the first adsorption column 206a in step S74. When the dew point in the outlet of the first adsorptioncolumn 206 a is judged by the first dew point meter 210 a to be equal toor greater than the set value (YES in step S75), then in step S76, aninstruction to replace the first adsorption column 206 a is produced.

Normal operations are further carried out using the second adsorptioncolumn 206 b (step S77). When the dew point in the outlet of the secondadsorption column 206 b is judged by the second dew point meter 210 b tobe equal to or greater than the set value (YES in step S78), then instep S79, an alarm is produced and the water electrolysis system 220 isstopped.

When an instruction to replace the first adsorption column 206 a isjudged to have been provided (NO) in step S72, it is determined that thefirst adsorption column 206 a is likely to experience breakthroughimmediately after starting of the water electrolysis system 220, andstep S80 is carried out. In step S80, the second adsorption column 206 bis used immediately after starting the water electrolysis system 220.Therefore, in the same manner as the seventh embodiment, an operationusing the second adsorption column 206 b is carried out for a set timet₂ (e.g., the set time t_(a)), and then a breakthrough judgment of thesecond adsorption column 206 b is started. Then, steps S77 to S79 arecarried out in the above manner.

As described above, in the eighth embodiment, when the second adsorptioncolumn 206 b is used immediately after starting the water electrolysissystem 220 in step S80, stopping of electrolysis in the waterelectrolysis system 220 is prevented for a set time t₂ after initiationthereof, in the same manner as the seventh embodiment. Therefore, thewater concentration can advantageously be maintained at the thresholdvalue (e.g., 5 ppm) or less in the hydrogen tank 204. Furthermore, sincethe dew point is detected by the second dew point meter 210 b after theset time t₂ has elapsed, the breakthrough judgment of the secondadsorption column 206 b can be carried out highly accurately.

In addition, in the eighth embodiment, even when a high dew point isdetected by the first dew point meter 210 a immediately after startingthe water electrolysis system 220, due to water being attached to theinside of the hydrogen supply pipe 222 a or the like, operation iscontinued regardless of the detection result, and then the dew point isdetected by the first dew point meter 210 a. Thus, an accurate judgmentcan be made as to whether or not breakthrough has occurred in the firstadsorption column 206 a.

While the invention has been particularly shown and described withreference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the spirit of the invention as defined bythe appended claims.

What is claimed is:
 1. A method for operating a water electrolysissystem, wherein the water electrolysis system contains: a high-pressurehydrogen production unit for electrolyzing water, thereby generatingoxygen at an anode side and generating hydrogen at a cathode side; ahydrogen storage unit for storing hydrogen discharged from thehigh-pressure hydrogen production unit; a hydrogen supply pipe forsupplying hydrogen generated in the high-pressure hydrogen productionunit to the hydrogen storage unit; and a water adsorption unit, which isconnected to the hydrogen supply pipe, for adsorbing water contained inthe hydrogen generated in the high-pressure hydrogen production unit,the method comprising the steps of: starting the water electrolysissystem; measuring an elapsed time from start of the water electrolysissystem; and preventing the electrolysis in the water electrolysis systemfrom stopping if the measured elapsed time is less than a set time, sothat the water electrolysis system continues the electrolysis at leastfor the set time, wherein the set time, during which stopping of theelectrolysis is prevented, is selected based on an initial waterconcentration in the hydrogen storage unit and a flow rate of hydrogenproduced, whereint _(a)={(C _(H2O) _(—) _(tank)−α)×V _(tank) _(—) _(std)}/{(α−C _(H2O)_(—) _(we))×F}; and wherein α is a variable defining a predeterminedthreshold value of water concentration, F is the flow rate of hydrogenproduct, t_(a) is the set time, C_(H2O) _(—) _(tank) is waterconcentration in the hydrogen storage unit, V_(tank) _(—) _(std) is thestandard-state volume of the hydrogen in the hydrogen storage unit, andC_(H2O) _(—) _(we) is the water concentration of the hydrogen flowproduct.
 2. A method for operating a water electrolysis system, whereinthe water electrolysis system contains: a high-pressure hydrogenproduction unit for electrolyzing water, thereby generating oxygen at ananode side and generating hydrogen at a cathode side; a hydrogen storageunit for storing hydrogen discharged from the high-pressure hydrogenproduction unit; a hydrogen supply pipe for supplying hydrogen generatedin the high-pressure hydrogen production unit to the hydrogen storageunit; and a water adsorption unit, which is connected to the hydrogensupply pipe, for adsorbing water contained in the hydrogen generated inthe high-pressure hydrogen production unit, the method comprising thesteps of: starting the water electrolysis system; measuring an elapsedtime from start of the water electrolysis system; and preventing theelectrolysis in the water electrolysis system from stopping if themeasured elapsed time is less than a set time, wherein the waterelectrolysis system further comprises: a water amount detection unit fordetecting an amount of water in the hydrogen supply pipe, and which isconnected to the hydrogen supply pipe on a downstream side of the wateradsorption unit; and an operation stop judgment unit for stoppingoperation of the water electrolysis system if the water amount detectedby the water amount detection unit is larger than a threshold value,wherein, after the measured elapsed time becomes equal to or greaterthan the set time, the operation stop judgment unit starts judgingwhether or not the water electrolysis system can be continuouslyoperated based on the water amount detected by the water amountdetection unit.
 3. The method for operating the water electrolysissystem according to claim 2, wherein: the water adsorption unit containsa first adsorption column and a second adsorption column, after startingof the water electrolysis system, the water electrolysis system isoperated normally using the first adsorption column within apredetermined time selected depending on a water-handling capacity ofthe second adsorption column, and after the predetermined time haselapsed, the water amount in the hydrogen supply pipe connected to thefirst adsorption column is detected by the water amount detection unit,which is connected to the hydrogen supply pipe adjacent to the firstadsorption column at a downstream side of the first adsorption column,and it is judged whether or not breakthrough of the first adsorptioncolumn has occurred based on the detected water amount.